In recent years, organic EL (Electro Luminescence) displays have been gaining attention as thin, lightweight, and fast-responsive display devices. While small-size organic EL displays have mainly been developed, development of medium-size and large-size organic EL displays is also conducted in recent years.
A TFT (Thin Film Transistor) substrate for small-size organic EL displays is manufactured using low-temperature polysilicon. In a manufacturing process using low-temperature polysilicon, both a P-channel type TFT and an N-channel type TFT can be formed on a TFT substrate. Accordingly, it is possible to suitably design a pixel circuit including an organic EL device using two types of TFTs, and to reduce wiring and power lines on the TFT substrate. In addition, a drive circuit for an organic EL device can be formed on the TFT substrate.
By contrast, a TFT substrate for medium-size and large-size organic EL displays is manufactured using amorphous silicon, microcrystalline silicon, or IGZO (Indium Gallium Zinc Oxide), in order to reduce cost. However, formation of a P-channel type TFT on a TFT substrate in a manufacturing process using such a material has not been successful at practical level so far. Therefore, in a medium-size or large-size organic EL display, it is necessary to configure a pixel circuit using only N-channel type TFTs.
Further, as it is not possible to form a P-channel type TFT on the TFT substrate, it becomes difficult to form a drive circuit for an organic EL device on the TFT substrate. As a result, ends of scanning lines are often pulled outside the TFT substrate as they are. In this case, as the number of scanning lines increases, the manufacturing cost is increased and reliability is reduced. Therefore, in medium-size and large-size organic EL displays, it is necessary to reduce the number of scanning lines as much as possible.
There have conventionally been known various pixel circuits for organic EL displays. For example, as shown in FIG. 9, Patent Document 1 describes a pixel circuit including N-channel type TFTs 80 to 84, capacitors 85 and 86, and an organic EL device 87. As shown in FIG. 10, Patent Document 2 describes a pixel circuit including P-channel type TFTs 90 to 95, a capacitor 96, and an organic EL device 97.